Processes for the production of EpB by the selective epoxidation of 1,3-butadiene in the presence of a modified silver catalyst are described in U.S. Pat. Nos. 4,897,498, 4,950,773, 5,138,077 and 5,362,890. The feed to the epoxidation zone comprises butadiene, an oxygen-containing gas and an inert diluent gas in various proportions. The gaseous epoxidation effluent typically comprises EpB, butadiene, oxygen, inert gas and a mixture of reaction by-products comprising water, carbon dioxide, acrolein, furan, vinylacetaldehyde and crotonaldehyde. After removal of the EpB product, e.g., as described in U.S. Pat. No. 5,117,012, most or all of the epoxidation effluent gas is recycled to the epoxidation zone. During the epoxidation process, some of the reactant butadiene and/or product EpB is decomposed to carbon dioxide and water. If not removed, the carbon dioxide will increase in the epoxidation effluent recycle stream, i.e., to at least 2 mole percent or more, affecting detrimentally the efficiency of the silver-catalyzed oxidation. For example, when using an epoxidation feed comprising butadiene, an oxygen-containing gas, an inert diluent gas and approximately 2 mole percent carbon dioxide, the EpB production rate can be reduced by as much as 50 percent as compared to a substantially identical epoxidation feed containing 0.002 mole percent carbon dioxide. Therefore, it is desirable, if not essential, that an EpB production facility includes means for the removal of CO.sub.2 from the epoxidation recycle gas stream.
In Kirk-Othmer Encyclopedia of Technology, 4th Edition, "Ethylene Oxide", 930-933 (1994), Dever et al. disclose that the standard gas-phase epoxidation process for the production of ethylene oxide from ethylene with pure oxygen is operated at a pressure of 2-3 MPa (290 to 435 psia) and that the recycle gas contains 5-10 mole percent carbon dioxide. Dever et al. further state that at carbon dioxide concentrations above 15 mole percent, catalyst activity is adversely affected. In such a process, the carbon dioxide partial pressure in the recycle gas is maintained at 0.1 to 0.3 MPa (14.5 to 43.5 psia) by contacting the ethylene oxide absorber off-gas with a carbon dioxide-absorbing solution in a second absorber. Zomerdijk and Hall, Cat. Rev. --Sci. Eng., 23(1&2) 163-185 (1981), Ozero, U.S. Pat. No. 4,879,396, and Ozero and Landau, Encyclopedia of Chemical Processing and Design, "Ethylene Oxide" 289-290 (1984) all teach that contact of the recycle gas with a hot potassium carbonate solution is the standard means for the removal of carbon dioxide from ethylene oxide plants.
As is disclosed by Kohl and Riesenfeld, Gas Purification, 4th Edition, 211-238 (1985), in such a process with the above-cited carbon dioxide partial pressures, the carbon dioxide-laden recycle gas typically is contacted with a lean hot potassium carbonate solution in a countercurrent absorption tower. The carbon dioxide in the gas reacts with the potassium carbonate solution in the absorber and is removed as a carbon dioxide-rich liquid stream from the bottom of the absorber. The chemically-bound carbon dioxide is stripped from the rich carbonate solution by a combination of heat and pressure changes and the lean absorber solution is recycled to the carbon dioxide absorber. For vapor phase, ethylene oxide production, carbon dioxide removal requirements for maintaining reactor productivity are not very demanding. The carbon dioxide removal system must be able to reduce the carbon dioxide concentration in the recycle gas stream from a feed partial pressure of around 0.1 MPa to 0.45 MPa (14.5 to 65.3 psia) to an outlet carbon dioxide partial pressure of at most 0.1 MPa to 0.3 MP (14.5 to 43.5 psia).
Tennyson and Schaaf, Oil & Gas J., 78-86 Jan. 10, (1977), disclose that the most economical process for achieving such carbon dioxide removal efficiencies given the above carbon dioxide absorber feed partial pressures and desired outlet carbon dioxide partial pressures is hot potassium carbonate solutions. Tennyson and Schaaf also state that below about 0.069 MPa (10 psia) carbon dioxide partial pressure in the feed gas to the carbon dioxide absorption system, hot potassium carbonate solutions become uneconomical for achieving carbon dioxide outlet partial pressures of about 0.002 MPa (0.3 psia), a level much lower than required for ethylene oxide production. Moreover, physical solvents for carbon dioxide absorption such as methanol, N-methylpyrrolidinone, and water are unsuitable for such demanding carbon dioxide removal requirements.
U.S. Pat. No. 5,312,931 discloses that the gas-phase epoxidation process for the production of 3,4-epoxy-1-butene (EpB) from 1,3-butadiene with pure oxygen is operated at a pressure of 0.2-0.9 MPa (29 to 130.5 psia) with 4 to 25 mole percent oxygen in the reactor feed (0.012 to 0.25 MPa--1.74 to 36.25 psia oxygen partial pressure), and that most preferably the recycle gas contains less than 0.5 mole percent carbon dioxide (typically 0.001 to 0.004 MPa carbon dioxide partial pressure). Thus, the carbon dioxide removal requirements for the recycle system in EpB production are much more stringent than ethylene oxide production, and outside of the normal economic range of the typical hot potassium carbonate system.
Kohl and Riesenfeld, Gas Purification, 4.sup.th Edition, 184-186 (1985), disclose that aqueous solutions of sodium hydroxide or potassium hydroxide can be used to reduce carbon dioxide levels in gas streams to very low levels, e.g., as low as 1 part per million by volume (ppmv) regardless of inlet carbon dioxide partial pressure. The sodium or potassium hydroxide reacts with the dissolved carbon dioxide to form heat-stable salts which cannot be decomposed at economical pressures and temperatures to regenerate the alkali metal hydroxide. Thus, the alkali metal hydroxide solutions cannot be recycled and large amounts of aqueous alkali hydroxides must be purchased and waste salt stream must be discarded, making such a process uneconomical for bulk removal of carbon dioxide such as for a butadiene epoxidation process.
Tennyson and Schaaf, Oil & Gas J., 78-86 Jan. 10, (1977), disclose that alkanolamine solutions such as monoethanol amine (MEA) can be used to achieve economically very low carbon dioxide outlet partial pressures, e.g., 0.00069 MPa--0.1 psia or lower, with feed carbon dioxide partial pressures in the same range or higher. Kohl and Riesenfeld, Gas Purification, 4.sup.th Edition, 129-133 (1985), disclose that such alkanolamine solutions are very susceptible to oxidative degradation and that oxygen should be excluded rigorously from the carbon dioxide absorber feed gases. Further evidence of the perceived detrimental effects of oxygen on alkanolamine solutions is presented in U.S. Pat. No. 3,137,654. This patent discloses that even a small amount of oxygen in a MEA solution will lead to oxidative degradation of the MEA and release of the degradation product ammonia into the carbon dioxide absorber outlet gas. Moreover, the degradation products of MEA and other alkanolamines were found to promote corrosion of the absorber and related equipment when constructed of less expensive carbon steel.
For removal of carbon dioxide from streams such as flue gases containing relatively low levels of oxygen, U.S. Pat. Nos. 4,440,731 and 4,477,419 disclose methods of reducing oxygen-induced degradation and corrosion by the addition of copper salts to the alkanolamine solutions. Such flue gases typically contain 2-5 mole percent oxygen at atmospheric pressure, e.g., oxygen partial pressures of 0.002 MPa to 0.005 MPa--0.29 to 0.73 psia, an order of magnitude lower oxygen partial pressure (e.g., about 0.056 MPa--8 psia) than the typical recycle gas in a vapor phase EpB process as disclosed in U.S. Pat. Nos. 5,312,931 and 5,362,890.